Conventionally, a heated roller fixing method has been typically adopted to a fixing device used in electrophotographic apparatuses such as copying machines and printers. According to the method, a recording material such as a recording sheet carrying an unfixed toner image is passed between a pair of heated and pressured rollers so that the toner image melts and is fixed onto the recording material.
However, a problem with the heated roller fixing method is that melted toner on the recording material is likely to induce sticking to the rollers (i.e., a so-called offset phenomenon). Especially, color electrophotographic apparatuses are susceptible to such a problem, since they use color toner, which is inferior to conventional black toner in the mold releasing property.
So, in order to prevent an offset phenomenon from occurring in fixing devices of present electrophotographic apparatuses, especially in those of color electrophotographic apparatuses, it is essential to apply an offset preventive agent (mold releasing agent) having low surface energy, such as silicone oil, to the surfaces of the rollers.
A conventional fixing device equipped with an oil applying device incorporates an oil applying felt, which typically works in the following manner: The oil in an oil tank is sucked up by a capillary phenomenon of the oil applying felt that is disposed to be, at the top end thereof, in contact with an oil applying roller and to be immersed, at the bottom end thereof, in the oil in the oil tank. The oil is thus applied onto the surface of the oil applying roller, and levelled to a predetermined thickness by an oil levelling blade that is pressed to the oil applying roller by a predetermined pressure. The oil on the oil applying roller is then transferred onto the surface of a fixing roller in a contacting portion between the oil applying roller and the fixing roller.
Since the oil levelling blade is required to resist heat and not to swell in silicone oil, the oil levelling blade is typically made of fluororubber. Nevertheless, an oil levelling blade made of fluororubber has drawbacks: (1) The fixing device needs a larger driving torque, (2) an enough amount of oil is not applied to prevent the offset phenomenon, since too large a part of the oil is scraped off by the edge portion of the blade, and (3) foreign bodies, such as felt fibers falling off the oil applying felt, paper powder, and offset toner, are likely to stick to the edge portion of the blade and disrupt levelled application of the oil.
Japanese Laid-Open Patent Applications No. 60-60673/1985 (Tokukaisho 60-60673) and No. 5-158371/1993 (Tokukaihei 5-158371) and address these problems and disclose methods of: (1) coating the blade surface with a fluororesin layer; (2) adhering a "Teflon" (product name for the tetrafluoroethylene manufactured by E. I. du Pont de Nemours and Co.) sheet onto the blade surface; and (3) sticking "Teflon" tape onto the blade surface.
However, as per the method of coating the blade surface with a fluororesin layer, despite the heat resistance temperature of the fluororubber constituting the blade being 260.degree. C., the fluororesin applied onto the blade surface needs to be baked at high temperatures, e.g. about 350.degree. C. for polytetrafluoroethylene (PTFE) and about 320.degree. C. for tetrafluoroethylene=perfluoroalkylvinylether copolymer (PFA). Such excess heat during baking degrades the fluororubber constituting the blade.
As per the method of adhering a "Teflon" sheet 80 onto the surface of a blade 56 constituted by a holder 56a and a blade segment 56b (see FIG. 5), the precision (evenness) of an edge portion A of the blade segment 56b declines and the oil is not uniformly applied, due to spillage of an adhesive agent 60 onto the edge portion A, non-uniform application of the adhesive agent 60 on the edge portion A, and/or other reasons. Such non-uniform application of oil leads to various undesirable results, including offset occurring where the oil is applied only in a less-than-required amount, and, if an image is printed on an OHP (overhead projector) sheet, a defective image with, for example, oily lines appearing on the OHP sheet.
As per the method of sticking "Teflon" tape onto the blade surface, the "Teflon" tape, which slides on the oil applying roller, wears thin and eventually cuts off in an edge portion due to insufficient durability thereof, and/or the "Teflon" tape, which sticks to the fluororubber only insufficiently, peels off the blade after a long use.